how good is the knight sack on f7 really?

I've seen some amateurs (i am one myself) And quite a couple of professional players play 1.Nf3 

2.Ng5 3.Nxf7. opening up your opponents king side, but some times the opponent gets mated quickly and sometimes they are able to defend the position.

Any ideas about when to do this and if this really is a good doubles opening?  

It is not good at all.

This is how newer players learn to attack but this attack rarely works against experienced players.

As long as no Queen comes you are pretty safe if you cover the squares around your king.

it usually is just an attack for inexperienced players

Move quickly, and if you see that something will mate you, tell your partner. You partner will have fun with his extra piece and probably has the luxury of sitting and letting you defend if necessary

So how to defend against this if someone plays this against me?

Play really fast, take the pieces your opponent sacks, and tell your partner no queen. people who play this sack really need your partner to be completely incompetent. if you just want to avoid it altogether play 1 ... e6 2 ... nc6 generally does the trick

@seahorses4ever I do e6 Nc6, but I've started noticing people push h4 and try for an early pawn storm, which proves annoying to defend (I mess that up a lot)

What are the key squares to defend there?

The sac on f7 is usless unless you have pieces to attack and in any case the e pawn hasn't moved or there isn't a piece defneding f7 and you have the right squares along the file. Lots of people saced on f7 against me and i still beat them. They just don't know the theory. Actually, there is none.

Also, it's best if your opponent doesn't get any pieces.

@Casper_J if your opponent pushes h4 your opponent will need a lot more in order to attack. depending on whether you have a N on c3, Bb4 can be effective since a5 becomes off limits for his pawn. ultimately let's say he uses a bunch of moves to push h4-h5-h6. that's basically the same scenario as if your opp placed p@h6 (pretty common).  he might be hoping you push h6, because that gives him h4-h5 then p@g6 for an attack on f7.  but also keep in mind that that h-pawn does some defensive work for white, too. he'll have a harder time dislodging a piece you put at g4, and if trades are plentiful p@g3 (with an eye toward p@f2 followed by a N check) can be dangerous for white.